Positron Emission Tomography (PET) is a medical imaging modality that allows studying metabolic processes of cells or tissues such as glucose transformation in energy. PET uses the coincident detection of two co-linear 511 keV photons emitted as a result of positron annihilation to reconstruct the spatial distribution of positron-emitting radiolabelled molecules within the body. Current PET human scanners can achieve 4-6 mm resolution and the scanner ring is large enough to let the patient occupy a relatively small portion of the field of view. On the other hand, small animal PET scanners have a smaller ring diameter (˜15 cm) and must achieve a higher resolution than their human counterpart (≦2 mm), which requires increasing the detector pixel density. In addition, because of the small diameter ring and large aspect ratio of long (˜2 cm) versus small-section (<4 mm2) detectors that are pointing toward the scanner center, an error occurs on the position of detection of the annihilation photons (511 keV) in the detector, as the depth of interaction of the photons in scintillators is not detected. For instance, depending on its entrance angle into the detector, a photon could be detected only in an adjacent detector. This error, which is known as parallax error, increases as the positron annihilation occurs further away from the scanner center because the Depth Of Interaction (DOI) within the crystals is not measured.
Various detector designs based on photomultiplier tubes (PMT) or Avalanche PhotoDiodes (APD) have been proposed so far. Most current PET scanners are based on arrays of single channel or dual-channel PMTs [1,2], position sensitive PMTs (PS-PMT) [3], or multi-channel PMTs (MC-PMT) [4,5] coupled to large matrices of scintillation crystals. PMTs are selected for their ease of use which results from their low noise contribution and high gain in PET data acquisition (DAQ) chain. However, PMTs are bulky and do not allow individual readout of scintillation crystals in densely packed arrays. When a 511 keV gamma ray hits one crystal in the matrix, the whole detector block is read out by several single-channel PMTs, or by one PS- or MC-PMT that gets blinded for the whole duration of this event (˜1 μs). The larger the crystal matrix and PMT, the lower is the overall count rate of the PET detector. Moreover, the decoding schemes that must be used to identify the crystal of interaction, based on the computation of the center of mass of the scintillation light pulse measured by several PMT channels, add some uncertainty to the actual position of detection. This technique limits the overall spatial resolution of PMT-based detectors. As a result, PMT-based PET detectors suffer of low count rate capabilities and reduced spatial resolution.
Due to these limitations, APD-based detection systems, which allow individual coupling of scintillation crystal to independent DAQ chains, have been considered for small animals PET scanners [6, 7, 8]. This approach has the advantage of increasing significantly the overall count rate per mm2 of detector area, but it suffers from the high cost of multiple, parallel DAQ channels and from electronic noise problems generated by APD photodetectors themselves.
The mechanical assembly of many PMT- or APD-based structures is still problematic and, while some have reached actual implementations, most did not go beyond simulation studies or laboratory tests. Due to the cost of materials and labor involved, complex mechanical structures can hardly be considered for implementation in industrial products.
Stacks of crystals with different scintillation light responses have been proposed as a simple means to improve DOI or position resolution without increasing the number of DAQ channels [9, 10, 11]. This approach can generally be implemented straightforwardly and can yield excellent results when appropriate signal processing methods are applied. Techniques for crystal identification can be based on pulse shape discrimination [12, 13, 14], statistical analysis [15, 16] or frequency domain transforms [17]. While fairly sophisticated pulse shape discrimination techniques can be easily implemented in analog or digital electronics, these methods perform poorly in noisy environment or with crystals having scintillation characteristics that are relatively similar. The more advanced statistical and frequency domain signal processing techniques that must be implemented in digital electronics generally achieve much superior performance, but are too computationally demanding to be carried out in real time.